Sensor cable design

cable. If I bring back the signal and ground wires in the same twisted pa ir, would that be considered symmetrical? More current would be flowing ou t to the sensor over the Vcc wire than would be returning through either th e signal or the ground wires.

les for cheap, in addition to maximal EMI performance both from external so urces and between pairs, and reduction of radiation from the cable signals. This one is $6.99 from Amazon, already connectorized:

5e-sftp

ence may not be.

Thanks for the refresher, but I already know quite a bit about driving and receiving controlled impedance transmission lines, and differential and com mon mode impedance. The OP needs to tell us the precision of the his A/D an d his ENOB spec at 5MHz so we can suggest drvr/rcvr pairs of adequate preci sion.

Not quite true: it just means that the 'transmission-line effects' are nearly indistinguishable from stray capacitance (if Z is high) or stray inductance (if Z is low).

So, the wiring can be an excess load, and cause frequency-dependent distortions. I had an audio cable that snaked through two or three rooms, and it performed very poorly at conducting line-level signals until I lowered the source impedance (to about 100 ohms); matching impedance gets rid of capacitive rolloff.

Sure, there's "expensive coax" ­ teflon-insulated silver-plated fancy stuff; There's also a good deal of cheap coax in the world ­ RG-59, for instance. Due to its use in cable TV, if you can't find a chunk for free, it's all of 6.75 cents/foot in bulk (without even shopping hard ­

7.4 cents for dual-shield) If you want shielded twisted pair, Cat 6 is over double the price (though it has more pairs than you need, so possibly you can find some STP that's actually cheaper than RG-59 - then again, there's at least a bulk market for Cat6, so perhaps not, too.)

You can even get RG-59 bonded with a pair of 18Ga power wires for 16.9 cents a foot (still not shopping all that hard, so you might be able to do better.)

RG-6 is also a comparatively inexpensive coax.

--
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Please don't feed the trolls. Killfile and ignore them so they will go away.

That's what I just said, if you restore the part that you snipped.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

"whit3rd"

** Correct - the cable is an electrically short transmission line, which if not terminated becomes be a pure C load. ** Simply reducing source Z to 100 ohms did the trick for your case - but it did not eliminate cable C.

.... Phil

Oops; omitted the other part of the solution. I also put a 100 ohm load on the distal end of the cable, thus applying the full transmission-line treatment. That was the easiest way to deal with cable C_stray (driving and driven ends both terminated into the characteristic impedance).

From memory, the characteristic impedance goes through some wild transitions determined by frequency. Something like 90 up to 120, or 120 down to 90, can't remember which.

Transferring energy either is no biggie, but transferring V(t) signal can become important

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Generally, as long as the OD of the center conductor, the ID of the 
shield, the concentricity between the center conductor and the shield 
and the dielectric constant of the insulation between the center 
conductor and the shield don't change, the impedance of the cable will 
remain constant and will suffer only a frequency dependent attenuation 
which increases as length and frequency increase.

I was referring to the twisted pair's characteristic impedance. However, since you described a coax, your comment that the characteristic impedance is constant is not true, unless you apply 'engineering' tolerances to it...The characteristic impedance of a coax changes with frequency.

For a 'word argument': Characteristic impedance is approximately the square root of the ratio of inductance per length over capacitance per length. At low frequencies, the whole center conductor's cross section is involved in carrying current and has a value of inductance per length. At high frequencies, skin effect rears its ugly head and you see the increased loss that you mentioned, but also the carriers are traveling along the outside of the center conductor, which changes the center conductor's inductance per length, thus the characteristic impedance of the coax changes with frequency.

At audio frequencies, the coaxial impedance can be several hundred ohms. At LF to UHF it is quite constant close to nominal impedance.

At extremely high frequencies the cable no longer act as a coaxial cable, but rather like a waveguide with different propagation modes depending on frequency.

At 10kHz, an RG-58 cable is what?

At 10kHz, an RG-59 cable is what?

In order to measure the impedance at 10 KHz, you need a chunk roughly

30 kilometers long. And that will have a center-conductor resistance of kilohms. So the characteristic impedance, including the resistive term, is huge. It's like twisted pairs, considered to be around 100 ohms for Ethernet but 600 ohms for telephone use.

It all falls out of the telegrapher's equation. As frequency tends towards zero, Z heads for root(R/G), which will generally be huge.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Huh? I guess you'd better define "coaxial impedance".

application, and I was wondering if anyone could check my design approach. Basically, my control circuit powers a wired sensor located about 2 meters away. The sensor circuit uses a single-ended op-amp to amplify a 5 MHz transducer signal, and then sends it back to the control circuit where it is digitized. So, I need 3 wires between the control circuit and the sensor: Vcc, ground, and signal.

other, I thought I should use twisted-triad wire to reduce cross-talk. External EMI shouldn't be a problem, so I don't think shielded cables are necessary. However, I haven't been able to find twisted-triad wire without a jacket or shield. What kind of wire should I use for this? If I used two twisted-pair wires for each sensor, what would I connect the extra wire to?

frequency content will be between signal and ground. Since I'm bringing back the signal and ground wires together, would this be considered a balanced circuit?

Cannonically, the two twisted pairs would be one pair for sensor power and one pair for sensor output. Bond return/ground wires of the pairs at the receiving point, and only maybe at the sensor.

?-)

Was waiting for someone to bring that up about the GND bonding location...

No you don't.

Open- and short-circuit impedance measurements on an electrically short sample are all you need.

I've done it using my HP4274A, on samples a couple of feet long.

In earlier life, I used to use a Marconi TF1275 Q-meter.

Zo = sqrt(R+jX/G-jB) = sqrt(R+jwL/G+jwC) = sqrt(Zsc/Yoc) = sqrt(Zsc.Zoc)

G is so small, you usually can't measure it, unless you have the most evil dielectric imaginable. It can be neglected.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

72.78 -j52.89, calculated from Belden 8262 published L, C and R figures. 79.05 -j25.29 (Belden 8263)

Any transmission line that has resistive loss must have a complex characteristic impedance, which is frequency dependent.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Only true in the case of lossless lines. Resistive losses mean a complex, frequency-dependent, characteristic impedance.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Well, you measured the L, R, and C, in separate open/short measurements, and calculated the impedance. My point was that impedance is a function of frequency, and it's not meaningful, as a transmission line impedance, if the line is a tiny fraction of a wavelength. There's no reason the OP should worry about line impedance or termination. Capacitive loading of the opamp could matter.

If I put a 10 foot coax into one black box, and the equivalent R, L, and C into another black box, as lumped elements, you couldn't tell the difference at 10 KHZ.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators